Conventionally, there has been proposed a D/A converter based on the 1-bit system as a device for converting a digital signal to an analog signal with high precision. This is achieved by improving the zero-cross distortion and linear degradation in signal conversion which occur in the conventional type of D/A converter based on the multi-bit system as described, for example, in the book All about Digital Audio (Sengaku Inoue, Denpa Shinbunsha).
Next, structure and a basic operation of the 1-bit system D/A converter based on the conventional technology is described with reference to FIG. 8. In FIG. 8, designated at the reference numeral 1 is a digital signal input terminal. 2a and 2b are adders, 3a, 3b and 3c are delaying units, 4 is a quantizing unit, 5 is a weighting section, 6 is a low-pass filter, 7 is an analog signal output terminal and 8 is a noise shaping circuit. Various configurations for this noise shaping circuit section 8 are know, however, a noise shaping circuit section 8 having three delaying units 3a, 3b, 3cand two adders 2a, 2b is considered.
In FIG. 8, at first, a digital signal having a plurality of bits is inputted into the digital signal input terminal 1. Then, this digital signal is converted to a pulse of .+-.1 in the quantizing unit 4 after passing through the adders 2a and 2b. This pulse is sent, after being converted to an analog signal by the low-pass filter 6, to the analog signal output terminal 7, and is, at the same time, sent to the weighting section 5, multiplied by a weight factor in the weighting section 5, sent to the delaying unit 3c, and is delayed by the delaying unit 3c, then the delayed digital signal is returned to the adders 2a, 2b respectively, and is added to the next digital signal in each of the adders 2a, 2b. The digital signal passing through the adder 2a is branched at a branch point X to be sent to the delaying unit 3a, delayed therein, returned to the adder 2a, and is added to the next digital signal in the adder 2a. Also, the digital signal passing through the adder 2b is branched at a branch point Y to be sent to the delaying unit 3b, delayed therein, returned to the adder 2b, and is added to the next digital signal in the adder 2b.
The 1-bit system D/A converter having the configuration as described above is a high-precision D/A converter with a comparatively simpler structure as compared to that of the multi-bit system of D/A converter based on the conventional technology. This is realized because of an effect of dispersing distribution of noise in the noise shaping circuit 8.
Although the noise shaping circuit 8 as described above plays an extremely important roll in the 1-bit D/A converter, there are some problems therein.
When a value held in each of the delaying units 3a, 3b becomes large immediately after the power is turned ON or due to noise entering therein, an overflow may occur in the noise shaping circuit because of a loop structure formed between the adders and delaying units. Further, the occurrence of this overflow may cause distortion in a waveform of an analog signal to be outputted.